The LPS-unresponsive C3H/HeJ mouse strain has been utilized to help elucidate the mode of action of bacterial endotoxin (LPS). Breeding experiments and genetic analysis demonstrate the existence of at least three distinct genes controlling LPS-responsiveness in mice, two autosomal and one X-linked. Chronological analysis suggests that a mutation resulting in a partial loss of LPS-responsiveness occurred in an autosomal gene between 1932 and 1940, and a second autosomal gene mutation that resulted in a complete loss of LPS-responsiveness occurred between 1960 and 1968. The failure of LPS to act as a mitogen in C3H/ HeJ mice has been found to be a defect intrinsic to the B-lymphocyte and not due to defective helper or suppressor activity in T-lymphocytes or macrophages. Interaction of LPS with B cells of C3H/HeJ mice results in a suppression of responsiveness instead of activation. Permanent LPS-responsive chimeras were made by adoptively transferring spleen cells from LPS-responsive, histocompatible C3H/HeN mice to x-irradiated C3H/HeJ mice. These chimeras demonstrated a heightened sensitivity to the lethal effects of LPS in vivo, suggesting that LPS-induced lethality is mediated by lymphoid cells.